Floyd Mayweather Jr's announcement that Devon Alexander is the front runner to fight him on May 4 in Las Vegas has been met with outrage by UK promoter Eddie Hearn.

Hearn's client Kell Brook was slated to fight Alexander as a mandatory challenger to his IBF belt on February 23 - until the American pulled out of the fight with a bicep injury on Monday.

Yet one day later Mayweather, 35, tweeted that his next fight would probably be with the 25-year-old Alexander.

"The negotiations for my fight are almost done," Mayweather wrote. "The front runner is IBF Champion Devon Alexander. It'd be a unification bout at welterweight."

Brook had said his bout with Alexander was one he had been waiting his whole life to contest while Alexander was open about his view that the Sheffield fighter was simply a stepping stone to bigger bouts.

Given the timing of the American's bicep injury and Mayweather's tweet, Hearn has questioned Alexander's real reasons for pulling out of the fight - one which had already been postponed once, by Brook.

But Hearn also said that the chances of Mayweather putting his WBC title on the line and Alexander chancing his IBF belt were slim, even if the fight does go ahead.

"Number one, we have a signed contract to fight Devon Alexander for the IBF title," Hearn said. "Number two, If they want Alexander v Mayweather to be a unification - they would have to ask for an exemption from the IBF which is very unlikely, given we already have signed contracts and any exemption would have to take place within 90 days of last bout.

"We questioned the injury and now we await news from Golden Boy which, given the news that they are looking to enter the UK promoting market, should be interesting. We will fight tooth and nail for Kell Brook and thanks for your support."